Composition comprising anti-bridging agent

ABSTRACT

The present invention provides powder coating compositions comprising at least one resin, at least one anti-bridging agent, and at least 40% w/w of pigment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part (CIP) of international application PCT/EP2005/013408 filed 8 Dec. 2005 which designated the U.S. and claims benefit of EP 04078582.6, dated 24 Dec. 2004, and is related to U.S. application Ser. No. (Unknown) filed even date herewith (Atty. Dkt. No. 4662-531) as the national phase entry in the U.S. of PCT/EP2005/013408, the entire content of each such application being hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a composition suitable for a powder coating composition comprising at least one resin and at least one anti-bridging agent. The invention further relates to a process for preparing such a composition, a powder coating composition, the coating and a substrate coated with the powder coating.

BACKGROUND TO THE INVENTION

Powder coating compositions usually contain a certain amount of pigment for aesthetic purposes. Some applications require high pigment loading in order to provide the necessary effect. This is especially the case where only a thin layer of coating composition is applied. For example, home furnishings, pre-coated metals, kitchen ware, and white goods are often coated with only a thin layer. However, once the pigment loading exceeds about 40% w/w, and particularly above 50%, the composition often has undesirable properties. For example, the flow, gloss, pigment-coverage and haze properties deteriorate.

It has been suggested that once the pigment load reached a certain level it could no longer be uniformly dispersed in the coating and hence caused unacceptable characteristics. It has also been suggested that poor pigment dispersion causes roughening of the coating surface and can lead to coatings with poor flow, low gloss and haze (Misev T. A, Powder Coatings, chemistry and Technology, John Wiley and Sons, Chichester, 1991).

There exists a need for a powder coating composition with high pigment loading which has acceptable physical properties.

SUMMARY OF THE INVENTION

The present invention provides to a powder paint composition comprising at least one resin and at least one anti-bridging agent.

As used herein, ‘high pigment load’ refers to pigment level of 40% w/w or greater, especially 45% w/w or greater, and particularly 50% w/w or greater.

As used herein, ‘pigment’ includes pigmented fillers, extenders and colorants.

While not wishing to be bound by theory, it is believed that the problems seen in high pigment loaded powder coating compositions are not caused by insufficient dispersion of the pigment particles as is generally believed. Indeed, surprisingly, the present inventors have found that the pigment dispersion is generally good which makes the powder coating composition susceptible to interactions between the resin and the pigment particles whereby the binder resin molecules bridge the pigment particles.

As used herein, the term “long wave roughness” refers to the root mean square roughness R_(q) of the coating surface determined from the topological features with a spatial wavelength of more than 5 micrometers. Long wave roughness may be measured by scanning the surface topography of the coated panels with a white light interferometer and ‘low-pass’ Fourier filtering of the measured surface as described in detail in the experimental section. The coatings herein preferably have a long wave roughness of 0.8 μm of less. Preferably, the coatings herein have a long wave roughness of 0.75μm, more preferably 0.7 μm, even more preferably 0.65 μm or less.

As used herein, the term “short wave roughness” refers in a similar way to the root mean square roughness R_(q) of the coating surface determined from the topological features with a spatial wavelength of less than 5 micrometers. As described in e.g. Hunt et al, J. Coat. Technol. Vol. 70, No. 880, p 45, (1998) this short wave roughness is known to be related to the quality of the pigment dispersion and to have a large effect on the optical properties of the cured coating like gloss and haze. For this reason, dispersants are sometimes used to improve the dispersion of the pigment particles in the powder paint.

As used herein, the term “coverage thickness” refers to the minimum thickness at which a particular coating composition provides sufficient pigment coverage to hide the substrate. Coverage thickness can be measured by preparing a standard aluminium test panel with a black coloured band of 2 cm width from top to bottom. The powder coating composition is applied to the panel so that there is a gradient in layer thickness from about 20 μm at the top to about 80 μm at the bottom. The coating is assessed in order to determine the point at which the black band is no longer visible. The thickness at this point is the coverage thickness. The coatings of the present invention preferably have a coverage thickness of 60 μm or less, more preferably 50 μm or less, even more preferably 45 μm or less.

The powder coating compositions according to the invention comprises at least one resin. The resin should be suitable for use in powder coating compositions. Examples of suitable resins include, but are not limited to, polyester resins, urethane resins, epoxy resins, acrylic resins, phenolic resins, polyesteramide resins, and combinations thereof.

Preferably, the resin has an acid or hydroxy value between 20 and 200 mg KOH/gram resin and more preferably between 20 and 120 mg KOH/gram resin.

The number average molecular weight (Mn in g/mol) of the resin is preferably between about 1,000 and about 7,000, more preferably between about 1,400 and about 6,000.

Preferably the resin is an amorphous solid at room temperature.

Preferably the resin has a viscosity lower than 200 Pa.s (measured at 160° C., Rheometrics CP 5), more preferably lower than 150 Pa.s. The glass transition temperature (T_(g)) of the resin is preferably greater than about 20° C., more preferably greater than about 35° C., even more preferably greater than about 45° C. The T_(g) of the resin is preferably less than about 100° C., more preferably less than about 85° C., even more preferably less than about 80° C. The T_(g) may be determined by differential scanning calorimetry (DSC) at a heating rate of 5° C./min.

The resin itself can be prepared in ways known to the man skilled in the art, see for example “Powder Coatings, Chemistry and Technology” by T. A. Misev, John Wiley and Sons, 1991, the whole book in general and Chapter 2 in particular, which is hereby incorporated by reference.

Preferably the resins for use herein are selected from polyester resins, acrylic resins, polyesteramide resins, epoxy resins, and combinations thereof. More preferably the resins for use herein are selected from polyester resins, acrylic resins, polyesteramide resins, and combinations thereof. Even more preferably the resin is selected from polyesters, more preferably from acid- and/or hydroxy-functional polyesters, even more preferably from carboxylic acid group-containing polyesters.

Among the suitable polyesters are those based on a condensation reaction of linear aliphatic, branched aliphatic and cyclo-aliphatic polyols with aliphatic, cyclo-aliphatic and/or aromatic poly carboxylic acids and anhydrides. The ratio of polyol and acids or anhydrides is such that there is an excess of acid or anhydride over alcohol so as to form a polyester which has free carboxylic groups.

Polyesters for use herein can comprise units of, for example, isophthalic acid, terephthalic acid, 2,6-naphthalene dicarboxylic acid, 4,4′-oxybisbenzoic acid, 3,6-dichloro phthalic acid, tetrachloro phthalic acid, tetrahydro phthalic acid, trimellitic acid, pyromellitic acid, hexahydro terephthalic acid (cyclohexane dicarboxylic acid), hexachloro endomethylene tetrahydro phthalic acid, phthalic acid, azelaic acid, sebacic acid, decane dicarboxylic acid, adipic acid, succinic acid, maleic acid, fumaric acid, and mixtures thereof. These acids may be used as such, or, in so far as available as their anhydrides, acid chlorides, and/or lower alkyl esters. Preferably, the polyester is based on at least one of isophthalic acid and/or terephthalic acid. Trifunctional or higher functional acids may be used also. Examples of suitable such acids include trimellitic acid or pyromellitic acid. These tri- or higher functional acids may be used as end groups or to obtain branched polyesters.

Hydroxy carboxylic acids and/or optionally lactones can also be used, for example, 12-hydroxy stearic acid, hydroxy pivalic acid and ε-caprolactone.

Monocarboxylic acids may also be used if desired. Examples of these acids are benzoic acid, tert.-butyl benzoic acid, hexahydro benzoic acid and saturated aliphatic monocarboxylic acids.

Useful polyalcohols, in particular diols, reactable with the carboxylic acids to obtain the polyester include aliphatic diols. Examples are ethylene glycol, propane-1,2-diol, propane-1,3-diol, butane-1,2-diol, butane-1,4-diol, butane-1,3-diol, 2,2-dimethylpropanediol-1,3 (neopentyl glycol), hexane-2,5-diol, hexane-1,6-diol, 2,2-bis-(4hydroxy-cyclohexyl)-propane (hydrogenated bisphenol-A), 1,4-dimethylolcyclohexane, diethylene glycol, dipropylene glycol, 2,2-bis[4-(2-hydroxy ethoxy)-phenyl] propane, the hydroxy pivalic ester of neopentyl glycol, 2-ethyl, 2-butyl propanediol-1,3 (butylethylpropane diol), 2-ethyl, 2-methyl propanediol-1,3 (ethylmethylpropane diol) and 2-methylpropanediol-1,3 (MP-Diol).

Tri- or higher functional alcohols may be used in order to obtain branched polyesters. Examples of suitable such polyols include glycerol, hexanetriol, trimethylol ethane, trimethylol propane, tris-(2-hydroxyethyl)-isocyanurate, penta erythritol and sorbitol.

The polyester may be prepared according to conventional procedures by esterification or transesterification, optionally in the presence of customary esterification catalysts for example dibutyltin oxide or tetrabutyl titanate. Preparation conditions and the COOH/OH ratio can be selected so as to obtain end products that have a particular desired acid number and/or a hydroxyl number.

The present coating compositions will usually comprise cross-linker. Any cross-linker or mixture of cross-linkers that is suitable for use (i.e. reactive with) with the resin may be used herein. Various types of cross-linkers or mixtures of cross-linkers may be used herein. Classes of suitable crosslinkers include epoxy resins, polyamides, (blocked) isocyanates, amino resins, polycarboxylic acids, acid anhydrides, polyphenols, Primid®-like compounds, and combinations thereof. Examples of suitable compounds include triglycidylisocyanurate (TGIC), polyisocyanates (for example caprolactam blocked IPDI (isophorone diisocyanate) derivatives, uretidione of IPDI, TDI (toluene diisocyanate) derivatives, TMXDI (tetramethylxylene diisocyanate) derivatives, or trimers thereof), polyphenols (for example polyphenols of the resol or novolac type), amino resins (for example alkylated melamine or benzoguanimine resins), alkylated melamine such as hexamethoxy-methylmelamine (HMMM), tryglycidil trimillitate, epoxidezed vegetable oil such as epoxidezed linseed oil and Epikote 828™, benzoguanamine (derivative), and combinations thereof.

It may be necessary to cure the composition comprising resin and crosslinker in order to form the coating. Examples of curing processes include thermal curing, curing with electromagnetic radiation, such as for example UV- or electron beam curing. Depending on the nature of the functional groups it is also possible to use two (dual-cure) or more types of curing processes.

The present compositions comprise at least one anti-bridging agent. As used herein, “anti-bridging agent” refers to a substance that decreases the bridging of pigment particles by the binder resin molecules. Preferred anti-bridging agents do this during the compound preparation and the early stages of powder paint flow and cure.

Preferred anti-bridging agents for use herein have the general formula X_(n)—Y_(m) wherein X is a pigment reactive group, Y is a resin soluble group, n is from 1-3 and m is from 1-4. Preferably n is 1 and m is from 1 to 3.

The reactivity or affinity of the X group(s) towards the pigment particles can be defined by measuring Langmuir adsorption isotherms. Resin, anti-bridging agent and pigment are mixed together in a suitable liquid medium. After a setn amount of time the pigment is filtered off and the amounts of the different components remaining in the liquid phase are measured. Thereby, the amount of the components which have adhered to the pigment surface can be calculated. The affinity of the X groups towards the pigment particles can be calculated in relation to the functional groups of the resin. If that affinity is high enough compared to the resin functional groups, the anti-bridging agent will preferentially attach to the pigment surface and prevent significant resin-pigment interaction.

The amount of the anti-bridging agent in the powder paint composition is preferably at least about 0.01 w/w %, more preferably at least about 0.1 w/w %, even more preferably at least about 0.3 w/w %, even more preferably still at least about 0.5 w/w % (based on the amount agent in the resin). The amount of anti-bridging agent is preferably about 15 w/w % or less, more preferably about 10 w/w % or less, even more preferably about 5 w/w % or less (based on the amount agent in the resin).

The anti-bridging agent herein preferably comprises at least one reactive group that has a high reactivity towards the surface of the pigment. Examples of suitable reactive groups include phosphoric acids, sulphuric acids, sulphonic acids, boronic acids, maleic anhydrides, and combinations thereof. More preferred reactive groups are phosphoric acids.

Preferably, the anti-bridging agent also comprises a second group that is reactive towards the resin and/or cross-linker. Preferably the anti-bridging agent comprises a reactive group with a similar reactivity as the binder resin and the cross-linker and a lower reactivity towards the pigment groups than the first reactive group. Examples of suitable reactive groups for this purpose include carboxylic acid, amine, epoxy, isocyanate and anhydride.

Preferably the present powder paint compositions (i.e. those comprising resin, pigment, and anti-bridging agent) have a stable viscosity at 200° C. That is, the anti-bridging agent reduces the amount by which the viscosity of a composition comprising resin and pigment changes when compared to a similar composition without the agent. As used herein, “stable viscosity” means that the once the composition is heated to 200° C. the viscosity changes by less than 25% over the course of 10 mins, more preferably by less than 10% over the course of 10 mins. The stability of the viscosity of the compound without a crosslinker was verified on a stress-controlled rheometer (MCR300, Paar-Physica) by applying a constant shear stress of 1 Pa for 1 hour. All measurements were performed at 200° C. and under a nitrogen atmosphere, using a parallel plate geometry (diameter and gap of respectively 25 mm and 1.5 mm) in shear.

The anti-bridging agent can be a solid or liquid at room temperature. Preferably the anti-bridging agent is a solid at room temperature.

The anti-bridging agent may be added to the resin at any time but is preferably added during the resin synthesis or while the resin leaves the reactor. With “during the resin synthesis” is meant that the anti-bridging agent is added before the resin leaves the synthesis reactor. For example, the anti-bridging agent may be added while the resin synthesis is occurring or after the synthesis but before the resin leaves the reactor. Generally the resin will be partially or fully cooled down before it leaves the synthesis reactor. Preferably the resin is partially cooled down before leaving the reactor. The anti-bridging agent is preferably added to the resin while the resin is partly cooled down just before the resin leaves the reactor. With “just before the resin leaves the reactor” is meant the moment in time where the resin has already all pre-determined properties and is more or less waiting to leave the reactor. Preferably the anti-bridging agent is added approximately 30 minutes preceding the resin leaving of the reactor.

Preferably the anti-bridging is added during the resin synthesis or while the resin leaves the reactor in which the resin is primarily produced. Alternatively, it may be added as separate component to the composition.

The present composition preferably comprises at least about 40 w/w % of pigment. Preferably the present compositions comprise at least about 45 w/w %, more preferably at least about 50 w/w % of pigment. Preferably the compositions herein comprise less than about 90 w/w %, more preferably less than about 75 w/w %, even more preferably less than about 60 w/w % of pigment.

The pigment for use herein can be of an inorganic or organic nature. As used herein “pigment” means a substance consisting of particles which impart colour to the composition. Pigments suitable for use in the present compositions include, for example, white pigments, pearlescent pigments, coloured pigments, black pigments, special effect pigments, and fluorescent pigments. Preferred pigments for use herein are inorganic. Preferred pigments for use herein include titanium oxides, zinc sulphides, zinc phosphates, iron oxides, chromium oxides, silicas, radium sulphates, calcium carbonates, and combinations thereof. More preferably the pigment herein comprises titanium dioxide.

Examples of commercially available pigments suitable for use herein include Kronos™ 2160, 2340, 2315, 3645, 2222, 2305 (available from Kronos); Ti-Pure™ 706, 960 (available from Du Pont™); Tiona 595 (available from Millennium™); Lithophone™ 30%; and combinations thereof.

The present coatings have a long wave roughness R_(q) of less than about 0.8 μm. It has surprisingly been found that coatings with lower long wave roughness show improved 20° gloss and lower haze.

The coatings herein preferably have a thickness of 80 μm or less, more preferably 60 μm or less, even more preferably 50 μm or less.

The coatings herein preferably have a gloss at 20° of 60 or better, more preferably of 70 or better, more preferably of 80 or better (as measured by Byk Gardner haze-gloss meter at a layer thickness of 601 μm).

The coatings herein preferably have a haze of 250 or less, more preferably of 150 or less, more preferably of 100 or less (as measured by Byk Gardner haze-gloss meter at a layer thickness of 60 μm).

The powder coating compositions herein generally comprises resin, crosslinker, pigment, and anti-bridging agent. Additionally other components can be added to the powder coating composition, for example flow control agents, catalysts, fillers, dispersants, light-stabilizers, biocides, and degassing agents.

The present powder coating compositions are preferably solid compositions that are suitable for application as a powder onto a substrate. With solid is here and hereinafter meant a compound that is solid at room temperature at atmospheric pressure. The glass temperature (Tg) of the powder coating composition preferably lies at or above 20° C. Preferably the Tg lies above 35° C., more preferably above 45° C.

The present invention also relates to a process for the preparation of a powder coating composition. The process may be any process suitable for producing a powder coating composition. Preferably the process comprises the steps:

-   -   (a) producing a resin comprising an anti-bridging agent;     -   (b) mixing the resin with pigment and crosslinker.

The present invention also relates to a process of applying a powder coating to a substrate wherein a composition comprising at least one resin, preferably at least one crosslinker, at least one anti-bridging agent, at least 40% w/w of pigment is applied to a substrate and cured.

The present invention also relates to a substrate fully or partly coated with a powder coating composition comprising at least one resin, preferably at least one crosslinker, at least one anti-bridging agent, at least 40% w/w of pigment.

The invention will further be elucidated by the following, non-limiting examples.

EXAMPLES

Uralac® P770 (available from DSM Resins, Zwolle, NL) was mixed with the ingredients listed in Table 1 in a pre-mixer (Kinematica Blender Microtron MB550), homoenised and then extruded in a double screw Prism extruder (Prism TSE 16 TSE (200 rpm, 120° C.) and sieved (retsch vibro (90 μm) to a powder with particle size less than 90 μm. The process was repeated with a similar amount of the base resin of Uralac® P770 which did not contain the anti-bridging agent. TABLE 1 powder paint compositions Base resin Uralac without anti GT- Resiflow P 770 bridging agent K2160 7004 PV-5 Benzoin parts parts Parts parts parts parts A 93 154.2 57 3 1.1 B 93 154.2 57 3 1.1 The powders were sprayed electrostatically on an aluminum substrate (AL-46). The coated substrates were cured for 10 minutes at 200° C. The coatings had a thickness of 60 μm. Haze and Gloss properties of the cured powder coatings are measured with a Byk Gardner haze-gloss meter at a layer thickness of 60 μm. Coverage thickness is measured by preparing a standard aluminium test panel with a black coloured band of 2 cm width from top to bottom.

The powders A and B are sprayed electrostatically to the panel so that there is a gradient in thickness from 20 μm at the top to 80 μm at the bottom. The coating thickness is assessed at the point at which the black band is no longer visible. The results are shown in Table 2. TABLE 2 Coating Properties Rq (long) Rq (short) Coverage μm nm Haze Gloss 20° μm A 0.53 29.6 106 85.6 40 B 1.92 38.2 533 33.9 65 The powders A and B are prepared without the presence of crosslinker and the stability of the viscosity of the compositions is verified on a stress-controlled rheometer (MCR300, Paar-Physica) applying a constant shear stress of 1 Pa for 1 hour. All measurements are performed at 200° C. and under a nitrogen atmosphere, using a parallel plate geometry (diameter and gap of respectively 25 mm and 1.5 mm) in shear. It is found that the viscosity of A increases by approximately 5%. The viscosity of B increases by more than 40%. 

1. Powder coating comprising: a. at least one resin; b. at least one crosslinker; c. at least one anti-bridging agent; d. at least 40% w/w of pigment; wherein the coating has an average thickness of 60 μm or less.
 2. Composition according to claim 1 comprising at least 50% w/w of pigment.
 3. Coating according to claim 1 wherein the coating has a thickness of from 20 μm to 60 μm.
 4. Powder coating composition comprising: a. at least one resin; b. at least one anti-bridging agent; c. at least 40% w/w of pigment; wherein the coating has a long wave roughness of 0.8 μm or less.
 5. Composition according to claim 4 wherein the gloss at 20° is 60 or better.
 6. Composition according to claim 4 wherein the haze is 250 or less.
 7. Powder paint composition comprising: a. at least one resin; b. at least one anti-bridging agent; c. at least 40% w/w of pigment; wherein the composition has a stable viscosity at a temperature of 200° C.
 8. Composition according to claim 7 wherein the anti-bridging agent contains at least one reactive groups selected from phosphoric acid, sulphuric acid, sulphonic acid, boronic acid, maleic anhydrides, and combinations thereof.
 9. A powder coating comprising an anti-bridging agent having an average thickness of 80 μm or less.
 10. A powder paint composition comprising an anti-bridging agent. 